Many of the resin molded products are molded by using one kind of material, but some of them are multilayer molded products of the same material or different kinds of materials in order to obtain strength or to add a special function such as, for examples electromagnetic wave shielding. It can be said that applying a surface coating to a molded product provides one type of the above-described multilayer molded products. As for the conventional art for obtaining such multilayer molded products, the following can be cited.
There is an injection molding method in which a number of movable metal molds of different sizes are provided for use with one fixed metal mold. In this method, the combinations of the metal molds are changed by successively chancing a movable metal mold of small size to that of a larger size for use with the one fixed metal mold, and by laying resin of the same material or different materials each time when the combination is changed, a multilayer product is obtained.
However, the entire structure becomes complicated by including a number of movable metal molds. The change of the movable metal molds cannot be carried out until a molded product has been solidified to some extent by cooling the resin; therefore, the total time for molding becomes longer. In addition, synchronous programs for each motion of the movable metal mold and for changing operation of the movable metal molds become complicated.
As for arts for surface coating, these include an art for coloring the surface of a finished molded product by a spray or a brush, an art for using colored resin for the top layer of a multilayer molded product molded based on the above-described conventional injection molding, method, and an insert injection molding art for carrying out injection molding by previously providing a ready-made colored sheet inside the metal molds.
However, surface coating by a spray or the like requires a coating device or manpower, and compatibility (mutually soluble property) cannot be secured on the boundary surface between the coating material and the molded product unless special coating material is used; therefore, the adhering strength of the coating material to the molded product and the bonding strength of the coating material itself become lower. The accuracy in coating, such as coating thickness, coating flatness, or the like, becomes lower. An insert injection molding art has an advantage that various kinds of patterns and designs can adhere to the surface of a molded product; but unless the metal molds are heated, compatibility on the boundary surface between the resin and a colored sheet cannot be expected; therefore, the adhering strength of the colored sheet to the molded product is low.
As for another surface coating art, there is known an injection molding method in which metal molds are slightly opened after a cooling step is completed (specifically, a condition in which a gap occurs inside the metal molds by the shrinkage of the molded product), coating material is injected from a coating nozzle provided in the metal molds, the molds are closed again, and the coating material in the gap is dried (refer to, for example, Japanese Patent Application Laid-open No. 6-198672). This is an improved method of an injection molding method in which the amount of closing is not controlled; while in the injection compression molding method, the amount of closing is freely controlled by a movable metal mold. Specifically, if the molds are closed after injecting the coating material, while the resin is still hot, by an ordinary injection molding method, there is no gap between the metal molds and the molded product, so that the coating material has no place to go. Therefore, the coating material and the resin, which are still in liquid form, leak to the outside of the metal molds, resulting in poor molding. The above-described improved method solves the disadvantage, and enables the surface coating.
However, in the above-described improved injection molding method, the time of the completion of the drying of the coating material is delayed as much as the time required for the resin to be cooled. Since the gap into which the coating material is injected is basically determined not by the amount of closing but by the amount of shrinkage, the leakage from the metal molds to the outside occurs if an excessive amount of the coating material is injected. On the other hand, if the amount of injected coating material is too small, irregular coating will occur. When the thickness of a molded product is uneven, the amount of shrinkage differs at every uneven point, so that the amount of the gap caused by the shrinkage varies; therefore, irregular coating occurs. Further, since coating material is injected after the resin is cooled, compatibility on the boundary surface between the resin and the coating material cannot be expected; therefore, there is a disadvantage of a lower adhering strength of the coating material to the molded product and a lower bonding strength of the coating material itself.